Simulink™ from The MathWorks, Inc. of Natick, Mass., is an example of a graphical modeling environment, specifically a block diagram environment. Simulink™ allows users to create a pictorial model of a dynamic system. Models generated in Simulink™ consist of a set of symbols, called blocks. Each block can have zero or more input ports, output ports, and states. Each block represents a dynamic system whose inputs, states, and outputs can change continuously and/or discretely at specific points in time. Since each block is a dynamic system, it is represented by a set of equations. The lines are used to connect the blocks' ports to one another, and represent time-varying quantities used by the block's equations.
A block is referred to as ‘atomic’ if its functional definition is outside the context of the model in which it is placed. Simulink™ has a set of predefined atomic blocks (e.g. Sum, Product, Gain), and the user can also create their own atomic blocks through user-written ‘S-functions’. Being atomic, S-functions' functional definitions are specified outside the context of the model, for example using C code or MATLAB ‘m’ code. A ‘composite’ block is a block whose functional definition is specified through the model, using sets of atomic and composite blocks. Simulink™ permits the user to specify ‘subsystems’, composite blocks whose definition consists of interconnected sets of predefined blocks, user-written S-functions, and other Simulink™ subsystems. Subsystems can be nested hierarchically, defining a ‘model hierarchy.’